Since a steel radial tire was developed by Michelin Corporation in the last half of 1940s, a steel cord reinforced pneumatic tire using steel cords as a reinforcing material for at least one of a carcass and a belt, increases its share of the market. Recently, along with a transition to a belted bias tire and a radial tire, the steel cord reinforced pneumatic tire significantly increases its share of the market and rapidly increases its share in a tire for a truck. In rubber products particularly requiring the strength such as conveyor belts, hoses and the like in addition to the steel cord reinforced pneumatic tire is used a composite material formed by covering a metal reinforcing material such as a steel cord with a rubber composition for the purpose of reinforcing rubber to improve the strength and durability.
In order that the rubber-metal composite material develops a high reinforcing effect to provide a reliability, a stable adhesion not depending on conditions of mixing, compounding, storage and so on is required between the rubber and the metal reinforcing material. In order to obtain such a composite material, there is widely used a so-called direct vulcanization adhesion, in which the metal reinforcing materials such as steel cords or the like plated with zinc, brass or the like are embedded in the rubber composition containing sulfur and adhered thereto at the same time as the vulcanization of rubber during the vulcanization by heating. Until now, there are made various investigations for improving the adhesion between the rubber and the metal reinforcing material in the direct vulcanization adhesion, particularly the humidity-aged adhesion.
For example, there is reported a rubber composition compounded with a resorcin or a resorcin-formaldehyde resin (hereinafter abbreviated as “RF resin”) obtained by the condensation of resorcin and formalin for the purpose of improving the humidity-aged adhesion (JP-A-2001-234140), in which the humidity-aged adhesion between the steel cord and the rubber is exponentially improved by compounding the RF resin.
However, the resorcin or the RF resin is poor in the compatibility with rubber because the polarity is very high, and the precipitation of the resorcin or the RF resin or so-called blooming is caused in accordance with conditions of mixing, compounding, storage and so on, so that there is a fear of damaging an appearance of the rubber product. Also, when the rubber composition is stored over a long time of period ranging from the compounding to the vulcanization adhesion, a problem of deteriorating the adhesion is caused by blooming, so that it is necessary to rapidly conduct the vulcanization adhesion of the rubber composition containing the resorcin or the RF resin, which may damage the productivity of the rubber product.
Furthermore, there is reported an adhesive material made from a mixed polyester having a resorcin skeleton with a weight average molecular weight of 3000-45000 (JP-A-7-118621). Although the mixed polyester having a high molecular weight is more compatible with the rubber as compared with the RF resin, the compatibility can not still be satisfied. Moreover, when the mixed polyester having a high molecular weight is compounded with rubber, the viscosity of the compounded rubber is increased to cause a problem of lowering the processability, and the humidity-aged adhesion is not sufficient.